Tio2-containing silica glass

ABSTRACT

The present invention is to provide a TiO 2 —SiO 2  glass having suitable thermal expansion properties as an optical member of an exposure tool for EUVL. The present invention relates to a TiO 2 -containing silica glass having a temperature, at which a coefficient of thermal expansion is 0 ppb/° C., falling within the range of 23±4° C. and a temperature width, in which a coefficient of thermal expansion is 0±5 ppb/° C., of 5° C. or more.

TECHNICAL FIELD

The present invention relates a TiO₂-containing silica glass(hereinafter referred to as “TiO₂—SiO₂ glass” in this specification),and in particular, to a TiO₂—SiO₂ glass to be used as an optical memberof an exposure tool for EUV lithography. The EUV (extreme ultraviolet)light as referred to in the invention means light having a wavelength ina soft X-ray region or a vacuum ultraviolet region, specifically lighthaving a wavelength of from about 0.2 to 100 nm.

BACKGROUND ART

In the photolithography technology, an exposure tool for manufacturingan integrated circuit by transferring a fine circuit pattern onto awafer has hitherto been widely utilized. With the trend toward a higherdegree of integration and a higher function of an integrated circuit,the refinement of the integrated circuit is advancing. The exposure toolis hence required to form a circuit pattern image with high resolutionon a wafer surface at a long focal depth, and shortening of thewavelength of an exposure light source is being advanced. The exposurelight source is further advancing from conventional g-line (wavelength:436 nm), i-line (wavelength: 365 nm) and a KrF excimer laser(wavelength: 248 nm), and an ArF excimer laser (wavelength: 193 nm) iscoming to be employed. Also, in order to cope with a next-generationintegrated circuit whose circuit line width will become not more than 70nm, an immersion lithography technique and a double exposure technique,each using an ArF excimer laser, are regarded as being leading. However,it is considered that even these techniques would be able to cover onlythe generation with a line width of up to 45 nm.

Under the foregoing technical trends, a lithography technique using, asan exposure light source, light having a wavelength of 13 nm torepresent EUV light (extreme ultraviolet light) is considered to beapplicable over generation of 32 nm and thereafter, and is attractingattention. The principle of image formation of the EUV lithography(hereinafter referred to as “EUVL”) is identical with that of theconventional lithography from the viewpoint that a mask pattern istransferred using a projection optical system. However, since there isno material capable of transmitting light therethrough in the EUV lightenergy region, a refractive optical system cannot be used. Accordingly,the optical systems are all reflecting optical systems.

The optical member of an exposure tool for EUVL includes a photomask anda mirror, and is basically configured of (1) a substrate, (2) areflective multilayer formed on the substrate and (3) an absorber layerformed on the reflective multilayer. For the reflective multilayer, anMo/Si reflective multilayer in which an Mo layer and an Si layer arealternately laminated is investigated; and for the absorber layer, Taand Cr are investigated. For the substrate, a material having a lowcoefficient of thermal expansion is required so as not to generate astrain even under irradiation with EUV light, and a glass having a lowcoefficient of thermal expansion or the like is investigated.

The TiO₂—SiO₂ glass is known as an extremely low thermal expansionmaterial having a coefficient of thermal expansion (CTE) lower than thatof a silica glass. Also, since the coefficient of thermal expansion canbe controlled by the TiO₂ content in glass, a zero-expansion glass whosecoefficient of thermal expansion is closed to 0 can be obtained.Accordingly, the TiO₂—SiO₂ glass involves a possibility as a material tobe used in an optical member of an exposure tool for EUVL.

According to the conventional preparation method of a TiO₂—SiO₂ glass,first of all, a silica precursor and a titania precursor are eachconverted into a gas phase and then mixed with each other. The mixturein a gas phase is introduced into a burner and thermally decomposed,thereby forming a TiO₂—SiO₂ glass particle. This TiO₂—SiO₂ glassparticle is deposited in a refractory container and melted thereinsimultaneously with the deposition, thereby forming a TiO₂—SiO₂ glass.

Also, Patent Document 1 discloses a method in which a TiO₂—SiO₂ porousglass body is formed and converted it into a glass body, and a masksubstrate is then obtained.

The optical member of an exposure tool for EUVL reaches a temperature ofabout 100° C. during the film formation of a reflective film or the likeat the time of its manufacture. Also, since EUV light with high energyis irradiated at the time of use in the exposure tool for EUVL, there isa concern that the temperature of the member locally rises.

For these reasons, it is desirable that the optical member of anexposure tool for EUVL has a wide temperature region where thecoefficient of thermal expansion is substantially zero. However, theconventional TiO₂—SiO₂ glasses have a narrow temperature region wherethe coefficient of thermal expansion is substantially zero, and hence,were insufficient for the use as an optical member of an exposure toolfor EUVL.

In order to solve the foregoing problems in the conventional techniques,the present inventors disclose, in Patent Document 2, a TiO₂—SiO₂ glasshaving a fictive temperature of not higher than 1,200° C., an Fconcentration of 100 ppm or more and a coefficient of thermal expansionat from 0 to 100° C. of 0±200 ppb/° C., and a method for manufacturingthe TiO₂—SiO₂ glass.

It had been thought that the TiO₂—SiO₂ glass is small in a change of acoefficient of thermal expansion relative to the temperature, namelywide in the temperature range where the coefficient of thermal expansionis substantially zero, is excellent in homogeneity of the coefficient ofthermal expansion and the mechanical properties in glass, and isextremely suitable as a raw material of the member which constitutes anoptical system to be used for EUVL.

Patent Document 1: US-A-2002/157421

Patent Document 2: JP-A-2005-104820

DISCLOSURE OF THE INVENTION

However, as is clear from FIG. 2 of Patent Document 2, morespecifically, as is clear from the comparison between Example 1 andExample 2 in FIG. 2, even in a TiO₂—SiO₂ glass having a fictivetemperature of not higher than 1,200° C. and an F concentration of 100ppm or more, the temperature dependence of a coefficient of thermalexpansion is different if the F concentration is different.

Also, as is clear from the comparison among Examples 3 to 5 of FIG. 2,though they are not concerned with an F-containing TiO₂—SiO₂ glass, thetemperature dependence of a coefficient of thermal expansion isdifferent if the fictive temperature is different.

Accordingly, though the TiO₂—SiO₂ glass disclosed in Patent Document 2has a wide temperature range where the coefficient of thermal expansionis substantially zero, if at least one of the F concentration and thefictive temperature is different, the temperature dependence of acoefficient of thermal expansion becomes different, and the temperatureregion where the coefficient of thermal expansion is substantially zerobecomes different.

In carrying out EUVL, the temperature in an exposure tool for EUVL isstrictly controlled. In the optical member of an exposure tool for EUVL,it is necessary that the coefficient of thermal expansion issubstantially zero under the strictly controlled temperature. However,with the TiO₂—SiO₂ glass disclosed in Patent Document 2, there may bethe case where the coefficient of thermal expansion is not substantiallyzero at the temperature in the exposure tool, and hence, the TiO₂—SiO₂glass disclosed in Patent Document 2 was not necessarily sufficient asthe optical member of an exposure tool for EUVL.

In order to solve the foregoing problems of the background arttechniques, an object of the invention is to provide a TiO₂—SiO₂ glasshaving suitable thermal expansion properties as an optical member of anexposure tool for EUVL. More specifically, an object of the invention isto provide a TiO₂—SiO₂ glass whose coefficient of thermal expansion atthe time of irradiation with EUV light is substantially zero when usedas an optical member of an exposure tool for EUVL.

In order to achieve the foregoing objects, the invention provides aTiO₂-containing silica glass (hereinafter referred to as “TiO₂—SiO₂glass of the invention”) having a temperature, at which a coefficient ofthermal expansion is 0 ppb/° C., falling within the range of 23±4° C.and a temperature width, in which a coefficient of thermal expansion is0±5 ppb/° C., of 5° C. or more.

It is preferred that the TiO₂—SiO₂ glass of the invention has a fictivetemperature of not higher than 850° C., a TiO₂ content of from 3 to 9%by mass and an OH concentration of 100 ppm or more.

It is preferred that the TiO₂—SiO₂ glass of the invention has a fictivetemperature of not higher than 850° C., a TiO₂ content of from 3 to 9%by mass and an F concentration of 1,000 ppm or more.

In the TiO₂—SiO₂ glass of the invention, a temperature range where acoefficient of thermal expansion is substantially zero is wide, and thetemperature region where a coefficient of thermal expansion issubstantially zero is coincident with the temperature of an opticalmember at the time of irradiation with EUV light. Therefore, theTiO₂—SiO₂ glass of the invention is extremely suitable as an opticalmember of an exposure tool for EUVL.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of plotting the relationship between CTE and thetemperature.

FIG. 2 is a graph of plotting the relationship between CTE and thetemperature, with respect to Examples 1 to 5.

FIG. 3 is a graph of plotting the relationship between CTE and thetemperature, with respect to Examples 6 and 7.

BEST MODE FOR CARRYING OUT THE INVENTION

The TiO₂—SiO₂ glass of the invention is hereunder described withreference to the accompanying drawings.

The TiO₂—SiO₂ glass of the invention has a temperature, at which acoefficient of thermal expansion (CTE) is 0 ppb/° C. (cross-overtemperature; COT), falling within the range of 23±4° C. and atemperature width ΔT, in which a coefficient of thermal expansion (CTE)is 0±5 ppb/° C., of 5° C. or more.

The COT and ΔT of TiO₂—SiO₂ glass can be determined by measuring acoefficient of thermal expansion (CTE) of the TiO₂—SiO₂ glass by a knownmethod, for example, by using a laser interferometric dilatometer in thetemperature range of from −150 to +200° C. and plotting the relationshipbetween CTE and the temperature as shown in FIG. 1.

In carrying out EUVL, for the purpose of preventing changes in dimensionand shape of an optical member such as a mirror relative to thetemperature, it is desirable for the optical member to be placed in anexposure tool for EUVL that the coefficient of thermal expansion at22±2° C. is 0±5 ppb/° C., i.e., substantially zero. It is more desirablethat for the optical member to be placed in an exposure tool for EUVLthat the coefficient of thermal expansion at 22±3° C. is 0±5 ppb/° C.,i.e., substantially zero. However, it has been suggested that thetemperature of the optical member locally rises, especially in the caseof a member close to a light source, because EUV light with high energyis irradiated.

Though it depends on an irradiation condition of EUV light, there may bethe case where the temperature of the optical member locally rises byabout 4 to 6° C. under a usual irradiation condition of EUV light incarrying out EUVL.

When the COT is in the range of 23±4° C., namely from 19° C. (i.e.,23-4° C.) to 27° C. (i.e., 23+4° C.), and the temperature range ΔT wherethe coefficient of thermal expansion is 0±5 ppb/° C. is 5° C. or more,the coefficient of thermal expansion of the optical member issubstantially zero under a temperature condition (22±2° C.) of theoptical member at the time of irradiation with EUV light. In thisspecification, the phrase “the coefficient of thermal expansion beingsubstantially zero” means that the coefficient of thermal expansionbeing 0±5 ppb/° C.

Also, in the TiO₂—SiO₂ glass of the invention, the ΔT is preferably 6°C. or more, and more preferably 7° C. or more. When the ΔT is 8° C. ormore, the coefficient of thermal expansion can be ±5 ppb/° C. in theforegoing temperature range of 23±4° C., and hence, the ΔT is especiallypreferably 8° C. or more.

The TiO₂—SiO₂ glass of the invention satisfying the foregoing COT and ΔTrequirements can be obtained by regulating either or both of the glasscomposition and the fictive temperature.

An embodiment of the TiO₂—SiO₂ glass of the invention satisfying theforegoing COT and ΔT requirements (hereinafter referred to as “TiO₂—SiO₂glass (1)”) satisfies the following requirements.

TiO₂ content: 3 to 9% by mass

Fictive temperature: not higher than 850° C.

OH concentration: 100 ppm or more

Accordingly, the TiO₂—SiO₂ glass (1) contains OH in addition to TiO₂ andSiO₂. In the TiO₂—SiO₂ glass (1), though the remainder exclusive of TiO₂and OH is SiO₂, other components than TiO₂, SiO₂ and OH may becontained.

It is known that the coefficient of thermal expansion of TiO₂—SiO₂ glassvaries with the concentration of TiO₂ to be contained (see, for example,P. C. Schultz and H. T. Smyth, in: R. W. Douglas and B. Ellis, AmorphousMaterials, Willey, New York, p. 453 (1972)).

Accordingly, it is possible to control COT of the TiO₂—SiO₂ glass bycontrolling the TiO₂ content of the TiO₂—SiO₂ glass. Specifically, theCOT is controlled such that when the fictive temperature of theTiO₂—SiO₂ glass is set lower, the TiO₂ content is made higher, whereaswhen the OH content of the TiO₂—SiO₂ glass is set higher, the TiO₂content is made higher. The TiO₂—SiO₂ glass (1) has a TiO₂ content offrom 3 to 9% by mass. When the TiO₂ content is less than 3% by mass orexceeds 9% by mass, the COT does not exist in the range of 23±4° C.Specifically, when the TiO₂ content is less than 3% by mass, the COT islower than 19° C. Also, when the TiO₂ content exceeds 9% by mass, theCOT exceeds 27° C. The TiO₂ content is preferably 5% by mass or more,and more preferably 6% by mass or more. Also, the TiO₂ content ispreferably not more than 8% by mass.

As described in Patent Document 2, the present inventors have found thatthe fictive temperature is correlated with the width of the temperaturerange of zero expansion, namely, the fictive temperature is correlatedwith ΔT, and more specifically, when the fictive temperature is high,the ΔT is narrow, whereas when the fictive temperature is low, the ΔT iswide.

Owing to the fictive temperature of not higher than 850° C., theTiO₂—SiO₂ glass (1) has ΔT of 5° C. or more. When the fictivetemperature exceeds 850° C., the ΔT is less than 5° C.; and there is aconcern that when the TiO₂—SiO₂ glass (1) is used as an optical memberof an exposure tool for EUVL, the coefficient of thermal expansion ofthe optical member at the time of irradiation with EUV light may be notsubstantially zero.

In order to obtain the TiO₂—SiO₂ glass (1) having a fictive temperatureof not higher than 850° C., a method of keeping a TiO₂—SiO₂ glass moldedarticle formed in a prescribed shape at a temperature of from 600 to1,200° C. for 2 hours or more, and then decreasing the temperature tonot higher than 700° C. at an average temperature-decreasing rate of notmore than 5° C./hr (These procedures are hereinafter referred to as“procedures (A)”) is effective. In the Examples as described below, itis shown that when a TiO₂—SiO₂ glass molded article was kept at 1,100°C. for 10 hours, subsequently subjected to temperature decrease to 500°C. at a rate of 5° C./hr and then allowed to stand for natural coolingaccording to the foregoing method, the obtained TiO₂—SiO₂ glass (1) hada fictive temperature of 840° C. When the temperature decrease iscarried out at a slower average temperature-decreasing rate, a lowerfictive temperature is attained. For example, when the temperaturedecrease is carried out at a rate of 1° C./hr, the fictive temperaturecan be 800° C. or lower.

The fictive temperature of the TiO₂—SiO₂ glass can be measured by knownprocedures. In the Examples as described below, the fictive temperatureof the TiO₂—SiO₂ glass was measured by the following procedures.

With respect to a mirror polished TiO₂—SiO₂ glass, an absorptionspectrum is obtained by an infrared spectrometer (Magna 760,manufactured by Nikolet Company was used in the Examples as describedbelow). In this measurement, a data taking interval is set up at about0.5 cm⁻¹, and an average value obtained by scanning 64 times is employedfor the absorption spectrum. In the thus obtained infrared absorptionspectrum, a peak observed in the vicinity of 2,260 cm⁻¹ is attributed toan overtone of stretching vibration by an Si—O—Si bond of the TiO₂—SiO₂glass. A calibration curve is prepared from a glass of the samecomposition having a known fictive temperature by using this peakposition, thereby determining the fictive temperature. Alternatively, areflection spectrum of the surface is measured in the same manner byusing the same infrared spectrometer. In the thus obtained infraredreflection spectrum, a peak observed in the vicinity of 1,120 cm⁻¹ isattributed to stretching vibration by an Si—O—Si bond of the TiO₂—SiO₂glass. A calibration curve is prepared from a glass of the samecomposition having a known fictive temperature by using this peakposition, thereby determining the fictive temperature. A shift of thepeak position by a change in the glass composition can be extrapolatedfrom the composition dependency of the calibration curve.

When the TiO₂—SiO₂ glass (1) is used as an optical member of an exposuretool for EUVL, it is important to make the TiO₂/SiO₂ composition ratioin the glass uniform, from the standpoint of reducing a variation of thecoefficient of thermal expansion in the glass.

In the TiO₂—SiO₂ glass (1), a variation of the fictive temperature ispreferably within 50° C., and more preferably within 30° C. When thevariation of the fictive temperature exceeds the foregoing range, thereis a concern that a difference in the coefficient of thermal expansionis generated depending upon the site.

In this specification, the “variation of the fictive temperature” isdefined as a difference between a maximum value and a minimum value ofthe fictive temperature within an area of 50 mm×50 mm in at least oneplane.

The variation of the fictive temperature can be measured as follows. Atransparent TiO₂—SiO₂ glass body formed in a prescribed size is slicedto form a TiO₂—SiO₂ glass block of 50 mm×50 mm×2.0 mm. With respect tothe 50 mm×50 mm plane of this TiO₂—SiO₂ glass block, by measuring afictive temperature at intervals of a 10 mm pitch according to theforegoing method, the variation of the fictive temperature of the formedTiO₂—SiO₂ glass body is determined.

In order to regulate the fictive temperature to not higher than 850° C.,it is preferred that the TiO₂—SiO₂ glass (1) has an OH concentration of100 ppm or more.

By the addition of OH, the structural relaxation of the glass isaccelerated so that it becomes easy to realize a glass structure havinga low fictive temperature. Therefore, for the purpose of lowering thefictive temperature of the TiO₂—SiO₂ glass, it is an effective measureto incorporate OH. By regulating the OH concentration of the TiO₂—SiO₂glass (1) to 100 ppm or more and carrying out the procedures (A), theTiO₂—SiO₂ glass (1) having a fictive temperature of not higher than 850°C. can be obtained. When the OH concentration is less than 100 ppm, ittakes a very long period of time to obtain a TiO₂—SiO₂ glass having afictive temperature of not higher than 850° C.

For the purpose of lowering the fictive temperature of the glass, the OHconcentration is regulated preferably to 200 ppm or more, and morepreferably to 400 ppm or more. For the purpose of more effectivelylowering the fictive temperature, for example, lowering the fictivetemperature without lowering the average temperature-decreasing rate inthe procedures (A), the OH concentration is preferably 900 ppm or more,and more preferably 1,000 ppm or more.

The OH concentration of the TiO₂—SiO₂ glass can be measured by using aknown method. For example, the OH concentration can be determined froman absorption peak at a wavelength of 2.7 μm through the measurement byan infrared spectrometer (see J. P. Williams, et al., American CeramicSociety Bulletin, 55(5), 524, 1976). The detection limit of this methodis 0.1 ppm.

As the method for manufacturing an OH-containing TiO₂—SiO₂ glass, thereare several processes as follows. As one example thereof, there is amanufacturing method in which a TiO₂—SiO₂ glass fine particle (soot)obtained by flame hydrolysis or thermal decomposition of an Si precursorand a Ti precursor serving as glass-forming raw materials is depositedand grown by a soot process, thereby obtaining a porous TiO₂—SiO₂ glassbody; and after treating the obtained porous TiO₂—SiO₂ glass body in awater vapor-containing atmosphere, it is heated to a densificationtemperature or higher in a water vapor-containing atmosphere and furtherheated to a transparent vitrification temperature or higher, therebyobtaining an OH-containing TiO₂—SiO₂ glass. Examples of the soot processinclude an MCVD process, an OVD process and a VAD process depending uponthe preparation manner.

The densification temperature as referred to in this specification meansa temperature at which the porous glass body can be densified to anextent that a void cannot be confirmed by an optical microscope. Also,the transparent vitrification temperature as referred to herein means atemperature at which a crystal cannot be confirmed by an opticalmicroscope, and a transparent glass is obtained.

Also, there is a manufacturing method in which an Si precursor and a Tiprecursor serving as glass-forming raw materials are hydrolyzed andoxidized in an oxyhydrogen flame at from 1,800 to 2,000° C., therebyobtaining an OH-containing TiO₂—SiO₂ glass. At that time, the OHconcentration is controlled by controlling the flame temperature or gasconcentration.

In the TiO₂—SiO₂ glass (1), so far as the variation of the fictivetemperature falls within 50° C., and the variation of the OHconcentration falls within 50 ppm, it enables the distribution ofcoefficient of thermal expansion to fall within 300 ppb/° C. within anarea of 50 mm×50 mm in at least one plane, and hence is suitable as anoptical member for an exposure tool for EUVL.

The distribution of the coefficient of thermal expansion of theTiO₂—SiO₂ glass can be measured by using a known method. For example, atransparent TiO₂—SiO₂ glass formed in a prescribed size is cut anddivided into TiO₂—SiO₂ glass small pieces of 15 mm×15 mm×1 mm, and therespective small pieces are measured for a coefficient of thermalexpansion, thereby determining the variation of the coefficient ofthermal expansion of a formed TiO₂—SiO₂ glass block.

For the purpose of manufacturing the TiO₂—SiO₂ glass (1), amanufacturing method including the following steps (a) to (e) can beadopted.

Step (a):

A TiO₂—SiO₂ glass fine particle obtained through flame hydrolysis of anSi precursor and a Ti precursor, each of which is a glass-forming rawmaterial, are deposited and grown on a substrate, thereby forming aporous TiO₂—SiO₂ glass body. The glass-forming raw material is notparticularly limited so far as it is a raw material capable of beinggasified. Examples of the Si precursor include silicon halides such aschlorides (for example, SiCl₄, SiHCl₃, SiH₂Cl₂, SiH₃Cl, etc.), fluorides(for example, SiF₄, SiHF₃, SiH₂F₂, etc.), bromides (for example, SiBr₄,SiHBr₃, etc.) and iodides (for example, SiI₄, etc.); and alkoxysilanesrepresented by RnSi (OR)_(4-n) (wherein R represents an alkyl grouphaving from 1 to 4 carbon atoms; and n represents an integer of from 0to 3). Also, examples of the Ti precursor include titanium halides (forexample, TiCl₄, TiBr₄, etc.); and alkoxy titaniums represented byR_(n)Ti(OR)_(4-n) (wherein R represents an alkyl group having from 1 to4 carbon atoms; and n represents an integer of from 0 to 3). Also, asthe Si precursor and the Ti precursor, a compound of Si and Ti such as asilicon titanium double alkoxide can be used.

A seed rod made by silica glass (for example, the seed rod described inJP-B-63-24973) can be used as the substrate. Also, the shape of thesubstrate to be used is not limited to a rod form but may be in a plateform.

Step (b):

The porous TiO₂—SiO₂ glass body obtained in the step (a) is subjected totemperature rise to a densification temperature in a watervapor-containing atmosphere, thereby obtaining an OH-containingTiO₂—SiO₂ dense body.

The densification temperature is in general from 1,250 to 1,550° C., andespecially preferably from 1,300 to 1,500° C. When the OH content is 600ppm or more, the viscosity of the glass is lowered, and thedensification temperature is lowered. Accordingly, the temperature ispreferably from 1,250 to 1,450° C., and especially preferably from 1,300to 1,400° C. As the water vapor-containing atmosphere, an inert gasatmosphere where a water vapor partial pressure (P_(H2O)) is from 10,000to 200,000 Pa is preferred. Helium is preferred as the inert gas. It ispreferred that the treatment is carried out at a pressure of from about10,000 to 200,000 Pa under such an atmosphere.

When it is intended to regulate the OH concentration to less than 200ppm, the treatment may be carried out in an inert gas atmosphere wherethe water vapor partial pressure is from 10,000 to 30,000 Pa; when it isintended to regulate the OH concentration to from 200 to 400 ppm, thetreatment may be carried out in an inert gas atmosphere where the watervapor partial pressure is from 20,000 to 50,000 Pa; when it is intendedto regulate the OH concentration to from 400 to 600 ppm, the treatmentmay be carried out in an inert gas atmosphere where the water vaporpartial pressure is from 30,000 to 80,000 Pa; and when it is intended toregulate the OH concentration to more than 600 ppm, the treatment may becarried out in an inert gas atmosphere where the water vapor partialpressure is 50,000 Pa or more. The term “Pa” as referred to in thisspecification means an absolute pressure, not a gauge pressure.

Also, in the step (b), it is preferred for attaining increasedhomogeneity of the TiO₂—SiO₂ dense body that after placing the porousTiO₂—SiO₂ glass body under a reduced pressure (preferably not more than13,000 Pa, and especially not more than 1,300 Pa), an inert gas and aninert gas containing a water vapor or a water vapor is introduced untila prescribed water vapor partial pressure is attained, so that theatmosphere contains a water vapor.

Furthermore, it is preferred for attaining increased homogeneity of theTiO₂—SiO₂ dense body that after keeping the porous TiO₂—SiO₂ glass bodyin a water vapor-containing atmosphere at room temperature or atemperature of not higher than the densification temperature, thetemperature is raised to the densification temperature.

Step (c):

The OH-containing TiO₂—SiO₂ dense body obtained in the step (b) issubjected to temperature rise to the transparent vitrificationtemperature, thereby obtaining an OH-containing transparent TiO₂—SiO₂glass body. The transparent vitrification temperature is usually from1,350 to 1,800° C., and especially preferably from 1,400 to 1,750° C.When the OH content is 600 ppm or more, the viscosity of the glass islowered, and the transparent vitrification temperature is lowered.Accordingly, the temperature is preferably from 1,350 to 1,750° C., andespecially preferably from 1,400 to 1,700° C.

As the atmosphere, an atmosphere of 100% of an inert gas such as heliumand argon, or an atmosphere containing, as a major component, an inertgas such as helium and argon, is preferred. The pressure may be areduced pressure or normal pressure. In the case of a reduced pressure,the pressure is preferably not higher than 13,000 Pa.

Step (d):

The OH-containing transparent TiO₂—SiO₂ glass body obtained in the step(c) is heated to a temperature of the softening point or higher andformed in a desired shape, thereby obtaining an OH-containing formedTiO₂—SiO₂ glass body. The forming temperature is preferably from 1,500to 1,800° C. When the forming temperature is lower than 1,500° C., sincethe viscosity of the OH-containing transparent TiO₂—SiO₂ glass body ishigh, self-weight deformation does not substantially proceed. Also, thegrowth of cristobalite which is a crystal phase of SiO₂ or the growth ofrutile or anatase which is a crystal phase of TiO₂ occurs, therebycausing so-called devitrification. When the forming temperature exceeds1,800° C., sublimation of SiO₂ cannot be neglected.

The step (c) and the step (d) can be carried out continuously orsimultaneously.

Step (e):

The formed TiO₂—SiO₂ glass body obtained in the step (d) is kept at atemperature of from 600 to 1,200° C. for one hour or more and thensubjected to an annealing treatment of decreasing the temperature to nothigher than 500° C. at an average temperature-decreasing rate of notmore than 5° C./hr, thereby controlling the fictive temperature of theTiO₂—SiO₂ glass. Alternatively, the formed TiO₂—SiO₂ glass body which isobtained in the step (d) and is at 1,200° C. or higher is subjected toan annealing treatment of decreasing the temperature to not higher than500° C. at an average temperature-decreasing rate of not more than 60°C./hr, thereby controlling the fictive temperature of the TiO₂—SiO₂glass. After decreasing the temperature to not higher than 500° C., theTiO₂—SiO₂ glass can be allowed to stand for cooling. In that case, theatmosphere is preferably an atmosphere of 100% of an inert gas such ashelium, argon and nitrogen, an atmosphere containing, as a majorcomponent, such an inert gas, or an air atmosphere; and the pressure ispreferably a reduced pressure or normal pressure.

For the purpose of attaining a lower fictive temperature, it iseffective to carry out cooling at a slower cooling rate in a temperatureregion in the vicinity of an annealing point or a strain point of theglass.

Specifically, in the case where the OH concentration of the TiO₂—SiO₂glass is 100 ppm or more, or in the case where the F content of theTiO₂—SiO₂ glass is 1000 ppm or more, the slowest cooling rate in thecooling profile of the step (e) is preferably not more than 5° C./hr,more preferably not more than 4° C./hr, further preferably not more than2.5° C./hr, especially preferably not more than 2° C./hr, and mostpreferably not more than 1.5° C./hr.

Further, in the case where the OH concentration of the TiO₂—SiO₂ glassis less than 100 ppm, or in the case where the F content of theTiO₂—SiO₂ glass is less than 1000 ppm, the slowest cooling rate in thecooling profile of the step (e) is preferably not more than 2.0° C./hr,more preferably not more than 1.5° C./hr, and further preferably notmore than 1.0° C./hr.

Here, temperature raising/decreasing steps that require 100 hours orlonger for a temperature change of within ±5° C. are regarded astemperature-holding steps. In the temperature-varying steps other thanthe above-mentioned temperature-holding steps, the averagetemperature-decreasing rate determined from a temperature decreaseexceeding 5° C. and the time necessary for the temperature decrease isdefined as a cooling rate. Of the cooling rates thus defined, the lowestcooling rate is referred to as the slowest cooling rate.

Another embodiment of the TiO₂—SiO₂ glass of the invention satisfyingthe foregoing COT and ΔT requirements (hereinafter referred to as“TiO₂—SiO₂ glass (2)”) satisfies the following requirements.

TiO₂ content: 3 to 9% by mass

Fictive temperature: not higher than 850° C.

F concentration: 1,000 ppm or more

Accordingly, the TiO₂—SiO₂ glass (2) contains F in addition to TiO₂ andSiO₂. In the TiO₂—SiO₂ glass (2), though the remainder exclusive of TiO₂and F is SiO₂, other components than TiO₂, SiO₂ and F may be contained.

It is possible to control the COT of the TiO₂—SiO₂ glass by controllingthe TiO₂ content of the TiO₂—SiO₂ glass. Specifically, the COT iscontrolled such that when the fictive temperature of the TiO₂—SiO₂ glassis set lower, the TiO₂ content is made higher, whereas when the Fcontent of the TiO₂—SiO₂ glass is set higher, the TiO₂ content is madelower.

The TiO₂—SiO₂ glass (2) has a TiO₂ content of from 3 to 9% by mass. Whenthe TiO₂ content is less than 3% by mass or exceeds 9% by mass, there isa concern that the COT does not exist in the temperature range of 23±4°C. Specifically, when the TiO₂ content is less than 3% by mass, the COTis lower than 19° C. (i.e., 23-4° C.). Also, when the TiO₂ contentexceeds 9% by mass, the COT exceeds 27° C. (i.e., 23+4° C.). The TiO₂content is preferably 4% by mass or more, and more preferably 5% by massor more. Also, the TiO₂ content is preferably not more than 8% by mass,more preferably not more than 7.5% by mass, and especially preferablynot more than 7.0% by mass.

Owing to the fictive temperature of not higher than 850° C. and the Fconcentration of 10,000 ppm or more, the TiO₂—SiO₂ glass (2) has ΔT of5° C. or more. When the fictive temperature exceeds 850° C., the ΔT isless than 5° C.; and there is a concern, though it depends on the COT ofthe glass, that when the TiO₂—SiO₂ glass (2) is used as an opticalmember of an exposure tool for EUVL, the coefficient of thermalexpansion of the optical member at the time of irradiation with EUVlight may be not substantially zero. Taking into account the fact thatthe lower the fictive temperature, the wider ΔT, the fictive temperatureis preferably not higher than 830° C., and more preferably not higherthan 800° C. In order to further widen ΔT, the fictive temperature ispreferably not higher than 780° C.

In order to obtain the TiO₂—SiO₂ glass (2) having a fictive temperatureof not higher than 850° C., a method of keeping a TiO₂—SiO₂ glass moldedarticle formed in a prescribed shape at a temperature of from 600 to1,200° C. for 2 hours or more, and then decreasing the temperature to500° C. at an average temperature-decreasing rate of not more than 5°C./hr (These procedures are hereinafter referred to as “procedures (B)”)is effective. In the Examples as described below, it is shown that whena TiO₂—SiO₂ glass molded article was kept at 1,000° C. for 10 hours,subsequently subjected to temperature decrease to 300° C. at a rate of5° C./hr and then allowed to stand for natural cooling according to theforegoing method, the obtained TiO₂—SiO₂ glass (2) had a fictivetemperature of 750° C.

When the TiO₂—SiO₂ glass (2) is used as an optical member of an exposuretool for EUVL, it is important to make the TiO₂/SiO₂ composition ratioin the glass uniform, from the standpoint of reducing a variation of thecoefficient of thermal expansion in the glass.

In the TiO₂—SiO₂ glass (2), a variation of the fictive temperature ispreferably within 50° C., and especially preferably within 30° C. Whenthe variation of the fictive temperature exceeds the foregoing range,there is a concern that a difference in the coefficient of thermalexpansion is generated depending upon the site.

In order to regulate the fictive temperature to not higher than 850° C.,the TiO₂—SiO₂ glass (2) has an F concentration of 1,000 ppm or more.

It has already been known that the addition of F affects the structuralrelaxation of the glass (Journal of Applied Physics, 91(8), 4886(2002)). According to this, by the addition of F, the structuralrelaxation time is accelerated so that it becomes easy to realize aglass structure having a low fictive temperature (first effect).Therefore, for the purpose of lowering the fictive temperature of theTiO₂—SiO₂ glass, it is an effective measure to add F. Also, it isconsidered that the addition of F gives rise to an effect for wideningthe range of the ΔT (second effect).

By regulating the F concentration of the TiO₂—SiO₂ glass (2) to 1,000ppm or more and carrying out the procedures (B), it is possible toobtain the TiO₂—SiO₂ glass (2) having a fictive temperature of nothigher than 850° C. When the procedures (B) are carried out under acondition that the F concentration is less than 1,000 ppm, it isdifficult to obtain a TiO₂—SiO₂ glass having a fictive temperature ofnot higher than 850° C., and the ΔT of the TiO₂—SiO₂ glass does notbecome 5° C. or more.

For the purposes of lowering the fictive temperature of the glass andmaking the ΔT wide, the F concentration is regulated preferably to 3,000ppm or more, more preferably to 5,000 ppm or more, and especiallypreferably to 7,000 ppm or more.

The F concentration can be measured by using a known method and, forexample, can be measured according to the following procedures. That is,a TiO₂—SiO₂ glass is melted by heating with anhydrous sodium carbonate,and distilled water and hydrochloric acid are added to the obtained meltin a volume ratio to the melt of 1, respectively, thereby preparing asample liquid. An electromotive force of the sample liquid is measuredby a radio meter using No. 945-220 and No. 945-468 (all of which aremanufactured by Radio Meter Trading Co., Ltd.) as a fluorine ionselective electrode and a reference electrode, respectively; and afluorine content is determined on the basis of a calibration curve whichhas been previously prepared using a fluorine ion standard solution(Nippon Kagaku Kaishi, 1972(2), 350). The detection limit of this methodis 10 ppm.

The fluorine-containing TiO₂—SiO₂ glass can be manufactured by employingthe same soot process as in the foregoing OH-containing TiO₂—SiO₂ glassor a direct process. However, in the soot process, fluorine-containingmaterials are used as an Si precursor and a Ti precursor serving asglass-forming raw materials, or an Si precursor and a Ti precursor aresubjected to flame hydrolysis or thermal decomposition in afluorine-containing atmosphere to obtain a fluorine-containing porousTiO₂—SiO₂ glass body, thereby obtaining a fluorine-containing TiO₂—SiO₂glass body. Also, in the direct process, fluorine-containing materialsare used as an Si precursor and a Ti precursor serving as glass-formingraw materials, or an Si precursor and a Ti precursor are hydrolyzed andoxidized in an oxyhydrogen flame at from 1,800 to 2,000° C. in afluorine-containing atmosphere, thereby obtaining a fluorine-containingTiO₂—SiO₂ glass body.

For the manufacture of the TiO₂—SiO₂ glass (2), a manufacturing methodincluding the foregoing steps (a) to (e) can be adopted. However, in thestep (b), the porous TiO₂—SiO₂ glass body is kept in afluorine-containing atmosphere at a temperature of not higher than thedensification temperature, thereby obtaining a fluorine-containingporous TiO₂—SiO₂ glass body. This fluorine-containing atmosphere ispreferably an inert gas atmosphere containing from 0.1 to 100% by volumeof a fluorine-containing gas (for example, SiF₄, SF₆, CHF₃, CF₄, C₂F₆,C₃F₈, F₂, etc.). It is preferred that the treatment in such anatmosphere at a pressure of from 10,000 to 200,000 Pa for from severaltens minutes to several hours is carried out at a high temperature ofnot higher than the densification temperature as described below. Also,when it is intended to lower the treatment temperature for obtaining thesame doping amount of fluorine, this can be attained by prolonging thetreatment time, specifically, keeping the porous TiO₂—SiO₂ glass bodyfor from 5 to several tens hours. In order to increase the transmittanceof the obtained glass, it is preferred to mix an oxygen gas in the heattreatment atmosphere.

The use of a temperature which is higher than the densificationtemperature is not preferred because the densification of the porousTiO₂—SiO₂ glass body proceeds so that it becomes hard to incorporatefluorine into the interior of the porous TiO₂—SiO₂ glass body.

For example, when SiF₄ is used as the fluorine-containing atmosphere,the treatment temperature and treatment time can be set as follows, inaccordance with the amount of fluorine to be doped on the porousTiO₂—SiO₂ glass body.

When it is intended to regulate the doping amount of fluorine at 1,000ppm or more and less than 3,000 ppm, this can be achieved by keeping theporous TiO₂—SiO₂ glass body in a gas atmosphere containing from 2 to 10%by volume of a fluorine-containing gas at from 500 to 1,000° C. for from2 to several tens hours. When it is intended to regulate the dopingamount of fluorine at from 3,000 to 7,000 ppm, this can be achieved bykeeping the porous TiO₂—SiO₂ glass body in an inert gas atmospherecontaining from 5 to several tens % by volume of a water vapor at from800 to 1,100° C. for from 2 to several tens hours. When it is intendedto regulate the doping amount of fluorine at more than 7,000 ppm, thiscan be achieved by keeping the porous TiO₂—SiO₂ glass body in an inertgas atmosphere containing from 5 to several tens % by volume of a watervapor at from 1,000° C. or higher for from 2 to several tens hours. Inorder to increase the transmittance of the obtained glass, it ispreferred to mix an oxygen gas in the heat treatment atmosphere.Alternatively, the glass body is kept in an oxygen-containing atmosphereat from 300 to 1,300° C. for from 5 to several tens hours to an extentthat it is not densified. This is made for the purpose of preventingcoloration of the glass in the sequent heat treatment. The concentrationof oxygen in the atmosphere is preferably from 1 to 100%, and for thepurpose of preventing coloration of the glass more surely, it is morepreferably from 20 to 100%.

In the case where fluorine is doped on a synthetic silica glass to besynthesized by a soot process as in the background art, it has beenpointed out that doping with fluorine at a high temperature generates anoxygen deficient defect, which causes a reduction of the lighttransmittance. However, when used for an optical member to be used in areflecting optical system, the reduction of the light transmittance doesnot matter. Accordingly, by treating at a temperature of not higher thanthe transparent vitrification temperature, it is possible to incorporatean extremely large amount of fluorine, and the doping amount of fluorinecan be several thousand ppm or more at maximum.

Furthermore, since fluorine can be uniformly doped on the porousTiO₂—SiO₂ glass body within a short period of time between the steps (a)and (b), it is preferred to place the TiO₂—SiO₂ glass body under areduced pressure (preferably not more than 13,000 Pa, and especiallypreferably not more than 1,300 Pa) and then introduce a mixed gas of afluorine-containing gas and an inert gas thereinto until the pressurereaches normal pressure, thereby making the atmosphere into afluorine-containing atmosphere.

Also, in the step (e), the formed TiO₂—SiO₂ glass body is kept at atemperature of from 600 to 1,200° C. for one hour or more and thensubjected to an annealing treatment of decreasing the temperature to nothigher than 500° C. at an average temperature-decreasing rate of notmore than 60° C./hr, thereby controlling the fictive temperature of theTiO₂—SiO₂ glass. Alternatively, the formed TiO₂—SiO₂ glass body which isobtained in the step (d) and is at 1,200° C. or higher is subjected toan annealing treatment of decreasing the temperature to not higher than500° C. at an average temperature-decreasing rate of not more than 60°C./hr, thereby controlling the fictive temperature of the TiO₂—SiO₂glass. After decreasing the temperature to not higher than 500° C., theTiO₂—SiO₂ glass can be allowed to stand for cooling. In that case, theatmosphere is preferably an atmosphere of 100% of an inert gas such ashelium, argon and nitrogen, an atmosphere containing, as a majorcomponent, such an inert gas, or an air atmosphere; and the pressure ispreferably a reduced pressure or normal pressure.

For attaining a lower fictive temperature, it is effective to carry outcooling at a slower cooling rate in a temperature region in the vicinityof an annealing point or a strain point of the glass. Specifically, inthe cooling profile of the step (e), the slowest cooling rate ispreferably not more than 5° C./hr, more preferably not more than 4°C./hr, further preferably not more than 2.5° C./hr, especiallypreferably not more than 2° C./hr, and most preferably not more than1.5° C./hr.

EXAMPLES

The present invention will be illustrated in greater detail withreference to the following Examples, but the invention should not beconstrued as being limited thereto. Examples 1, 2 and 6 are inventionexamples, and the remainder is comparative examples.

Example 1

TiO₂—SiO₂ glass fine particles obtainable by gasifying each of TiCl₄ andSiCl₄ which are glass-forming raw materials of a TiO₂—SiO₂ glass andthen mixing and subjecting the mixture to heat hydrolysis (flamehydrolysis) in an oxyhydrogen flame was deposited and grown on asubstrate, thereby forming a porous TiO₂—SiO₂ glass body (step (a)).

Since it is hard to handle the obtained porous TiO₂—SiO₂ glass body asit is, the obtained porous TiO₂—SiO₂ glass body was kept in air at1,200° C. for 6 hours together with the substrate, and then separatedfrom the substrate.

Thereafter, the porous TiO₂—SiO₂ glass body was placed in anatmosphere-controllable electric furnace and the pressure was reduced to10 Torr at room temperature. Thereafter, water was boiled at atmosphericpressure and 100° C. in a glass-made bubbler, and the resulting porousTiO₂—SiO₂ glass body was kept in this atmosphere at 1,000° C. undernormal pressure for 4 hours while introducing a water vapor togetherwith an He gas into the furnace, thereby effecting doping with OH.

Thereafter, after raising the temperature to 1,450° C. in the sameatmosphere, the system was kept at this temperature for 4 hours, therebyobtaining an OH-containing TiO₂—SiO₂ dense body (step (b)).

The obtained OH-containing TiO₂—SiO₂ dense body was heated to 1,700° C.in an argon atmosphere using a carbon furnace, thereby obtaining anOH-containing transparent TiO₂—SiO₂ glass body (step (c)).

The obtained OH-containing transparent TiO₂—SiO₂ glass body was heatedto a temperature of the softening point or higher (1,750° C.) and formedin a desired shape, thereby obtaining an OH-containing formed TiO₂—SiO₂glass body (step (d)).

The obtained glass was kept at 1,100° C. for 10 hours and then subjectedto temperature decrease to 500° C. at a rate of 5° C./hr, followed byallowing it to stand for natural cooling (step (e)).

In Example 1, OH was incorporated into the glass body in step (c), andthe fictive temperature of the glass body was lowered by therate-cooling in step (e). Therefore, the COT was controlled byincreasing TiCl₄ in step (a) so as to give a higher TiO₂ content of theglass body, as compared to a glass body not containing OH and having ahigher fictive temperature.

Example 2

TiO₂—SiO₂ glass fine particles obtainable by gasifying each of TiCl₄ andSiCl₄ which are glass-forming raw materials of a TiO₂—SiO₂ glass andthen mixing and subjecting the mixture to heat hydrolysis (flamehydrolysis) in an oxyhydrogen flame was deposited and grown on asubstrate, thereby forming a porous TiO₂—SiO₂ glass body (step (a)).

Since it is hard to handle the obtained porous TiO₂—SiO₂ glass body asit is, the obtained porous TiO₂—SiO₂ glass body was kept in air at1,200° C. for 4 hours together with the substrate, and then separatedfrom the substrate.

Thereafter, the porous TiO₂—SiO₂ glass body was placed in anatmosphere-controllable electric furnace and the pressure was reduced to10 Torr at room temperature. Thereafter, the resulting porous TiO₂—SiO₂glass body was kept in this atmosphere at 1,100° C. under normalpressure for 4 hours while introducing a mixed gas of He and SiF₄ in aratio of 90/10 (by volume), thereby effecting doping with fluorine.

Thereafter, the system was kept in an atmosphere of 100% O₂ at 1,050° C.under normal pressure for 4 hours, and the temperature was then raisedto 1,450° C. in an atmosphere of 100% He, followed by keeping at thistemperature for 4 hours, thereby obtaining a fluorine-containingTiO₂—SiO₂ dense body (step (b)).

The obtained fluorine-containing TiO₂—SiO₂ dense body was heated to1,650° C. in an argon atmosphere using a carbon furnace, therebyobtaining a fluorine-containing transparent TiO₂—SiO₂ glass body (step(c)).

The obtained fluorine-containing transparent TiO₂—SiO₂ glass body washeated to a temperature of the softening point or higher (1,750° C.) andformed in a desired shape, thereby obtaining a fluorine-containingformed TiO₂—SiO₂ glass body (step (d)).

The obtained glass was kept at 1,000° C. for 10 hours and then subjectedto temperature decrease to 300° C. at a rate of 5° C./hr, followed byallowing it to stand for natural cooling (step (e)).

In Example 2 also, taking into considerations that the fictivetemperature of the grass body was lowered by the rate-cooling in step(e) and that fluorine was incorporated into the glass body in step (c),the COT was controlled by controlling the TiO₂ content of the glass bodyin step (a).

Example 3

TiO₂—SiO₂ glass fine particles obtainable by gasifying each of TiCl₄ andSiCl₄ which are glass-forming raw materials of a TiO₂—SiO₂ glass andthen mixing and subjecting the mixture to heat hydrolysis (flamehydrolysis) in an oxyhydrogen flame was deposited and grown on asubstrate, thereby forming a porous TiO₂—SiO₂ glass body (step (a)).

Since it is hard to handle the obtained porous TiO₂—SiO₂ glass body asit is, the obtained porous TiO₂—SiO₂ glass body was kept in air at1,200° C. for 4 hours together with the substrate and then separatedfrom the substrate.

Thereafter, the porous TiO₂—SiO₂ glass body was placed in anatmosphere-controllable electric furnace and the pressure was reduced to10 Torr at room temperature. Thereafter, the resulting porous TiO₂—SiO₂glass body was kept in this atmosphere at 900° C. under normal pressurefor 4 hours while introducing a mixed gas of He and SiF₄ in a ratio of90/10 (by volume), thereby effecting doping with fluorine.

Thereafter, the system was kept in an atmosphere of 100% O₂ at 1,050° C.under normal pressure for 4 hours, and the temperature was then raisedto 1,450° C. in an atmosphere of 100% He, followed by keeping at thistemperature for 4 hours, thereby obtaining a fluorine-containingTiO₂—SiO₂ dense body (step (b)).

The obtained fluorine-containing TiO₂—SiO₂ dense body was heated to1,700° C. in an argon atmosphere using a carbon furnace, therebyobtaining a fluorine-containing transparent TiO₂—SiO₂ glass body (step(c)).

The obtained fluorine-containing transparent TiO₂—SiO₂ glass body washeated to a temperature of the softening point or higher (1,750° C.) andformed in a desired shape, thereby obtaining a fluorine-containingformed TiO₂—SiO₂ glass body (step (d)).

The obtained glass was kept at 1,100° C. for 10 hours and then subjectedto temperature decrease to 300° C. at a rate of 150° C./hr, followed byallowing it to stand for natural cooling (step (e)).

Example 4

TiO₂—SiO₂ glass fine particles obtainable by gasifying each of TiCl₄ andSiCl₄ which are glass-forming raw materials of a TiO₂—SiO₂ glass andthen mixing and subjecting the mixture to heat hydrolysis (flamehydrolysis) in an oxyhydrogen flame was deposited and grown on asubstrate, thereby forming a porous TiO₂—SiO₂ glass body (step (a)).

Since it is hard to handle the obtained porous TiO₂—SiO₂ glass body asit is, the obtained porous TiO₂—SiO₂ glass body was kept in air at1,200° C. for 4 hours together with the substrate and then separatedfrom the substrate.

Thereafter, the porous TiO₂—SiO₂ glass body was placed in anatmosphere-controllable electric furnace and the pressure was reduced to10 Torr at room temperature. Thereafter, the temperature was raised to1,450° C. in an atmosphere of 100% He, followed by keeping at thistemperature for 4 hours, thereby obtaining a TiO₂—SiO₂ dense body (step(b)).

The obtained TiO₂—SiO₂ dense body was heated to 1,750° C. in an argonatmosphere using a carbon furnace, thereby obtaining a transparentTiO₂—SiO₂ glass body (step (c)).

The obtained transparent TiO₂—SiO₂ glass body was heated to atemperature of the softening point or higher (1,750° C.) and formed in adesired shape, thereby obtaining a formed TiO₂—SiO₂ glass body (step(d)).

The obtained glass was kept at 1,100° C. for 10 hours and then subjectedto temperature decrease to 500° C. at a rate of 150° C./hr, followed byallowing it to stand for natural cooling (step (e)).

Example 5

ULE#7972, manufactured by Corning Incorporated, which is known as azero-expansion TiO₂—SiO₂ glass.

Example 6

TiO₂—SiO₂ glass fine particles obtainable by gasifying each of TiCl₄ andSiCl₄ which are glass-forming raw materials of a TiO₂—SiO₂ glass andthen mixing and subjecting the mixture to heat hydrolysis (flamehydrolysis) in an oxyhydrogen flame was deposited and grown on asubstrate, thereby forming a porous TiO₂—SiO₂ glass body (step (a)).

Since it is hard to handle the obtained porous TiO₂—SiO₂ glass body asit is, the obtained porous TiO₂—SiO₂ glass body was kept in air at1,200° C. for 6 hours together with the substrate and then separatedfrom the substrate.

Thereafter, the porous TiO₂—SiO₂ glass body was placed in anatmosphere-controllable electric furnace and the pressure was reduced toabout 1,000 Pa (7.50 Torr) at room temperature. Thereafter, water wascharged in a glass-made bubbler, boiled at atmospheric pressure and 100°C., and then subjected to bubbling with an He gas, and the resultingporous TiO₂—SiO₂ glass body was kept in this atmosphere at 1,000° C.under normal pressure for 4 hours while introducing a water vaportogether with an He gas into the furnace, thereby effecting doping withOH.

Thereafter, after raising the temperature to 1,450° C. in the sameatmosphere, the system was kept at this temperature for 4 hours, therebyobtaining an OH-containing TiO₂—SiO₂ dense body (step (b)).

The obtained OH-containing TiO₂—SiO₂ dense body was heated to 1,700° C.in an argon atmosphere using a carbon furnace, thereby obtaining anOH-containing transparent TiO₂—SiO₂ glass body (step (c)).

The obtained transparent TiO₂—SiO₂ glass body was heated to atemperature of the softening point or higher (1,750° C.) and formed in adesired shape, thereby obtaining a formed TiO₂—SiO₂ glass body (step(d)).

The obtained glass was kept at 1,100° C. for 10 hours and then subjectedto temperature decrease to 900° C. at a rate of 10° C./hr, temperaturedecrease to 700° C. at a rate of 1° C./hr and subsequent temperaturedecrease to 500° C. at a rate of 10° C./hr, followed by allowing it tostand for natural cooling (step (e)).

In Example 6, OH was incorporated into the glass body in step (c), andthe fictive temperature of the glass body was lowered by therate-cooling in step (e). Therefore, the COT was controlled byincreasing TiCl₄ in step (a) so as to give a higher TiO₂ content of theglass body, as compared to a glass body not containing OH and having ahigher fictive temperature.

Example 7

TiO₂—SiO₂ glass fine particles obtainable by gasifying each of TiCl₄ andSiCl₄ which are glass-forming raw materials of a TiO₂—SiO₂ glass andthen mixing and subjecting the mixture to heat hydrolysis (flamehydrolysis) in an oxyhydrogen flame is deposited and grown on asubstrate, thereby forming a porous TiO₂—SiO₂ glass body (step (a)).

Since it is hard to handle the obtained porous TiO₂—SiO₂ glass body asit is, the obtained porous TiO₂—SiO₂ glass body is kept in air at 1,200°C. for 6 hours together with the substrate and then separated from thesubstrate.

Thereafter, the porous TiO₂—SiO₂ glass body is placed in anatmosphere-controllable electric furnace and the pressure is reduced toabout 10 Pa at room temperature. After raising the temperature to 1,450°C. in the vacuum atmosphere, the system is kept at this temperature for4 hours, thereby obtaining a TiO₂—SiO₂ dense body (step (b)).

The obtained TiO₂—SiO₂ dense body is heated to 1,750° C. in an argonatmosphere using a carbon furnace, thereby obtaining a transparentTiO₂—SiO₂ glass body (step (c)).

The obtained transparent TiO₂—SiO₂ glass body is heated to a temperatureof the softening point or higher (1,750° C.) and formed in a desiredshape, thereby obtaining a formed TiO₂—SiO₂ glass body (step (d)).

The obtained glass is kept at 1,100° C. for 10 hours and then subjectedto temperature decrease to 500° C. at a rate of 3° C./hr, followed byallowing it to stand for natural cooling (step (e)).

In Example 7, the fictive temperature of the glass body is lowered bythe rate-cooling in step (e). Therefore, the COT is controlled byincreasing TiCl₄ in step (a) so as to give a higher TiO₂ content of theglass body, as compared to a glass body not containing OH and having ahigher fictive temperature.

The temperature dependence of coefficient of thermal expansion of eachof the glasses of the foregoing Examples 1 to 7 is shown in FIGS. 2 and3. The coefficient of thermal expansion of glass was measured by using alaser interferometric dilatometer (LIX-1, manufactured by ULVAC-RIKO,Inc.).

Also, the results of the measurement of respective physical propertiesare summarized and shown in Table 1. With respect to the evaluationmethods, the measurements were made in accordance with the methodsmentioned above, respectively. Also, the COT shown in Table 1 wasderived by determining the temperature at which the coefficient ofthermal expansion was 0 ppb/° C., from the curve shown in FIGS. 2 and 3.The ΔT shown in Table 1 was derived by determining the temperature rangewhere the coefficient of thermal expansion was from −5 to 5 ppb/° C.from the curve of FIGS. 2 and 3.

TABLE 1 Fictive OH F temperature concentration concentration COT ΔT CTEat 22 ± 2° C. [° C.] [ppm] [ppm] [° C.] [° C.] [ppb/° C.] Example 1 8401030 N.D. 20.6 6.8 −0.9 to 4.8 Example 2 750 <10 9000 19.7 9.7 0.3 to4.3 Example 3 1020 <10  900 21.1 4.7 −2.3 to 6.2 Example 4 1070 30 N.D.24.4 4.1 −10.7 to −0.7 Example 5 900 880 N.D. −2.4 4.7 44.4 to 51.6Example 6 780 1030 N.D. 26.3 7.8 −8.7 to −3.1 Example 7 970 30 N.D. 24.44.1 −10.7 to 0.7

As is clear from Table 1, in Examples 1, 2 and 6 in which the COT fallswithin the range of 23±4° C., and the ΔT is 5° C. or more, thecoefficient of thermal expansion is substantially zero under atemperature condition (22±2° C.) in an exposure tool at the time ofcarrying out EUVL, and therefore, the glasses of these Examples 1, 2 and6 are suitable for an optical member of an exposure tool for EUVL.

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

This application is based on Japanese Patent Application Nos.2007-336603 (filed Dec. 27, 2007) and 2008-207705 (filed Aug. 12, 2008),and the contents thereof are herein incorporated by reference.

1. A TiO₂-containing silica glass having a temperature, at which acoefficient of thermal expansion is 0 ppb/° C., falling within the rangeof 23±4° C. and a temperature width, in which a coefficient of thermalexpansion is 0±5 ppb/° C., of 5° C. or more.
 2. The TiO₂-containingsilica glass according to claim 1, having a fictive temperature of nothigher than 850° C., a TiO₂ content of from 3 to 9% by mass and an OHconcentration of 100 ppm or more.
 3. The TiO₂-containing silica glassaccording to claim 1, having a fictive temperature of not higher than850° C., a TiO₂ content of from 3 to 9% by mass and an F concentrationof 1,000 ppm or more.